The present invention relates to a dry resist, and more particularly a high-quality dry resist capable of imparting high resolution and preventing occurrence of image fault.
Dry resist is popularly used in production of printed microcircuits. Dry resist is usually a laminated structure composed of a base film, a photoresist layer and a protective film. As a base film, a polyester film having excellent mechanical and optical properties, chemical resistance, heat resistance, dimensional stability, flatness and other features is mostly used. The photoresist layer is composed of a photosensitive resin, and the protective film is composed of a polyethylene or polyester.
The usage of the dry resist is briefly explained below.
First, the protective film is stripped off and the exposed photoresist layer is attached tightly on a conductive substrate bonded to a substrate. The conductive substrate is usually a copper plate. Then, a glass plate having microcircuits printed thereon is attached tightly to the base film side of the dry resist and light is irradiated thereto via the glass plate. As light passes through the transparent portion of the circuit image printed on the glass plate, the exposed portion alone of the photosensitive resin of the photoresist layer is reacted. Then, the glass plate and the base film are removed, and the unexposed portion of the photoresist layer is eliminated with an appropriate solvent. Etching is carried out with an acid or other suitable substance to remove the conductive substrate portion exposed by removal of the photoresist layer. This is followed by exposure to light and the reacted photoresist layer is eliminated by a suitable method. Consequently, the circuit of the conductive substance is formed on the substrate.
Recently, in the printed microcircuits, the circuits formed are very complicated, the lines are fine and close in interval, and accordingly high-level reproducibility of image formation and resolution are required. High quality is also required of the polyester film used as a base film.
It is essential that the polyester film used as a base film has a high transparency and a low haze. In the dry resist, when the photoresist layer is exposed to light, light passes through the base film as explained above. Therefore, if the base film is low in transparency, there arise the problems such as insufficient exposure of the photoresist layer and reduced resolution due to scatter of light.
On the other hand, the polyester film used as a base film is required to have an appropriate of slipperiness. Such slipperiness is necessary for bettering handling property of the polyester film when producing dry resist by forming photoresist layer on the said base film or handling property of the dry resist itself. Usually, this requirement is fulfilled by containing particles in polyester film so as to form fine protuberances on the film surface. Such protuberances are also closely adhere between the polyester film and circuit-printed glass substrate and eliminate the risk of impairing reproducibility of circuit image by reason that bubbles get in between the polyester film and the glass plate.
When protuberances are formed by particles contained in the film, however, transparency of the film is impaired. Thus, the development of a high-quality dry resist which can satisfy the property requirements for transparency, the slipperiness and an air leak property at the same time has been desired.
As a result of the present inventors' earnest studies, it has been found that when using as a base film a biaxially oriented laminated polyester film whose surface layer on at least one side contains particles having an average diameter of 0.01 to 3.0 .mu.m, and has a center line average roughness (Ra) of not less than 0.005 .mu.m and a maximum height (Rt) of less than 1.5 .mu.m, and which has a haze of not more than 1.5%, the produced dry resist is of high-quality and capable of imparting a high resolution and preventing occurrence of image fault. The present invention has been attained on the basis of this finding.